Method of coloring stainless steel



Patented Feb. 23, 1943 METHOD OF COLORING STAINLESS STEEL Clements Batcheller, Glens Falls, N. Y.

No Drawing. Application November 30, 1939, Serial No. 306,973

9 Claims.

My invention relates to the chemical coloration of the surface and subsurface zones of alloy steels and irons of the type commonly called stainless and generally containing 6% or more, by weight, of chromium.

In my United States Patent No. 2,172,353 and in my copending applications for Letters Patent Serial No. 206,043, filed May 4, 1938, and Serial Nos. 241,990 and 241,991, filed November 23, 1938, the later of which issued October 29, 1940, as U. S. Patent No. 2,219,554, I have described processes for the coloration of stainless steels by immersion in a solution of concentrated sulphuric acid and water in which one or more of a large number of etching-inhibiting-oxidizing agents are employed in a quantity sufllcient to prevent etching of the steel during treatment in the acid. By these processes a very beautiful deep black, color may be imparted to the steel, but the depth or thickness of the color film which is apparently in and substantially integral with the surface of the steel rather than a mere superficial coating thereon is very slight; the maximum thickness being about 1500 Angstrom units.

While these processes for coloring stainless steels are very simple and entirely practicable, the color films produced are not much harder or substantially more resistant to abrasion than the steel itself. This characteristic seriously affects the wide application of my processes which otherwise would, include many stainless steel surfaces that must withstand periodic cleaning and polishing with scouring compounds and the like.

My investigations, including chemical analyses of the color films, indicate that this lack of resistance to abrasion is largely due to the fact that the color films, although composed of oxides of the various elements in the steel, are not true oxides of these elements. I find that the softness of such color films and their susceptibility to damage or removal by abrasives is due to the fact that they contain a certain percentage of chemical water (water of combination). The presence of water in the films not only makes 1 them somewhat soft, or at least not substantially harder than the steel itself, but also somewhat porous.

I have found that such color films can be made extremely hard and resistant to abrasion, and their porosity very substantially reduced by the rather simple expedient of dehydrating them by heat treating the colored steel to a degree sulficient to drive off the water in the ,color film and also, apparently, to eflect a shrinking of the films which eliminates their initial porosity.

This may be accomplished without substantial change in color. In addition to hardening the film and very substantially increasing its resistance to abrasion I find that the resistance of the steel to the standard salt spray test and also to the corrosive action of various media, including the gases resulting from the combustion of explosives and priming compositions used in fire arms ,and ordnance, is remarkably enhanced.

The steel is first colored by treating it, preferably by immersion in a solution of concentrated sulphuric acid and water in which there is dissolved a sufficient quantity of an appropriate etching-inhibiting-oxidizing agent to prevent etching of the surface of the steel during the coloring treatment. The sulphuric acid solutions which I employ are comparatively highly concentrated and of such strength that if used without an etching inhibitor would immediately attack and quickly dissolve the steel. The solutions which I employ may contain from about 23 to about 54 parts, by weight, of sulphuric acid (1.84 sp. g.) and from about parts to about 64 parts, by weight, of water. In other words, the solution of acid and water may have an acid concentration (by weight) varying in round numbers from about 25% to about 65%, but for the best results I prefer to use a solution having an acid concentration, by weight, varying from about to about of the acid-water solution.

In this acid-water solution I dissolve a sufiicient quantity of an appropriate etching-inhibiting-oxidizing agent to prevent etching of the steel'surface at the temperature at which the coloring process is to be conducted. Generally speaking, the quantity of etching inhibitor necessary will increase with the acid concentration and also with the temperature, and it is possible to use such a large proportion thereof as to prevent satisfactory coloration. In the wider ranges of acid concentration given above, from about 4 to about 25 parts, by weight, of inhibitor may be used, but I prefer to use the acid, water and inhibitor in about the following proportions:

Parts by weight Etching-inhibiting-oxidizing agent 10-14 Sulphuric acid (1.84 sp. g.) 36-50 Water 40-50 In the above it Will be noted that the acid constitutes from about 40% to about 55%, by weight,

of the acid-water solution and that the quantity of inhibitor is comparatively small. Generally speaking the quantity of inhibitor should not greatly exceed the quantity necessary to prevent etching, and the minimum quantity can be readily determined for any acid concentration and any temperature by testing specimens of the steel to be colored in the acid-water solution at the temperature to be employed, and adding the inhibitor until attack on the steel ceases.

As an inhibitor I prefer to usesodium or potas- The reaction between the sulphuric acid and any of the ehromates and dichromates forms chromlc acid in the solution; the addition of a vanadate or metavanadate to the sulphuric acid solution forms vanadic or metavanadic acid; and the addition of a manganate or permanganate forms manganic or permanganic acid. Thus, it may be said that the etching-inhibiting-oxidizing agent which I employ is such as will add to the sulphuric acid-water solution a second acid selected from the group consisting of chromic, vanadic, meta'vanadic, manganic and permanganic. In other words, the coloring solution, by reason of the addition thereto of any of the above substances which, for the want of a more appropriate term, I have called etching-inhibiting-oxidizing agents will contain either chromic, vanadic, metavanadic,manganic or permanganic acid.

Generally speaking, the coloration treatment may be carried out with the bath at any temperature between normal atmospheric temperature (say 70 F.) and the boiling point of the solution. However, with the lower temperatures, the time required to produce a satisfactory color may run into hours or even days. On the other hand, with temperatures inexcess of about 220 1". the coloration takes place so rapidly that it is difficult to control and, moreover, the color film which is formedlis not particularly satisfactory or apparently integral with the surface of the steel in that it may be readily rubbed off. The temperatures which I prefer to employ are between about 180 F. and about 210 F. In this range the best color films are produced, the time required is comparatively short and therefore commercially practicable, and the color changes are not sufliciently rapid to prevent proper control. The steel should be removed from the solution when it has attained a black coloration. The time required depends on the acid concentration, the quantity of inhibitor, the temperature of the solution and the analysis of the steel, and is therefore somewhat indefinite, but with the precent. After coloring, the steel may be simply washed and dried.

The texture or finish of the colored surface depends upon the initial texture or finish of the steel. That is to say, if the steel before coloring is highly polished, this high polish will be maintained and even enhanced by the coloration treatment. On the other hand, if the initial finish of the steel is dull, the colored steel will be a dull black. j z

Where the steel is to be used in fire arms or for army ordnance a dead, flat, nonrefiectlng black is essential and this color can be best obtained treated by my process, are now being used in cerj tain ordnance, cannot bev evenly colored to a dead, non-reflecting black when given any of the usual pre-treatment acid pickles. However, by

subjectingthe surface to an abrasive treatment, such as blasting it with a mud composed of fine sand and water, its passivity is reduced or destroyed and a uniform, dead, non-reflecting black color can be produced by my process.

After coloring, the parts are simply washed and dried and then subjected to a simple furnace heating at a temperature sufilcient to effect a disassociation of the water combined therewith, preferably in the range of from about 800 F. to about 1200 F., and cooling. Although there may be a substantial change in the color, it is to be understood that the steel may be heated to as high as 1700 F. with no substantial injury to the. color film, and, when heated within the preferred temperature range the specimens lose only a verylittle of their intense black color but the surface. becomes hard and hi hly resistant to abrasion and corrosive media, particularly to the corrosive action of nitrogen-containing compounds, picric' acid and mercury-containing compounds result-- I of various ing from the combustion or explosion powders, fulminates, etc.

WhatIclaimis:

oxidizing agent selected from the group of acidsconsisting of chromic, vanadic, -metavanadic,"

manganic and permanganic, drying said film and thereafter dehydrating said oxides by heating the steel to a temperature of the order of 800 to 1700'F., and cooling, to harden said surface increase its resistance to abrasion. 1

2. The method of hardening the surface of a corrosion-resisting alloy steel containing at least 6%, by weight, of chromium and increasing itsresistance to corrosion and abrasion which comprises forming in said surface a thin film of dark color comprising hydrated oxides of iron and chromium by treating the steel in a solution of sulphuric acid, water and an etching-inhibiting-.

oxidizing agent selected from the group of acids For example, forgings of stainand consisting of chromic, vanadic, metavanadic, manganic and permanganic, drying said film and thereafter dehydrating said oxides by heating the steel to a temperature of the order of 800 to 1200 F., and cooling, to harden said surface and increase its resistance to abrasion.

3. The method of hardening the surface of a corrosion-resisting alloy steel containing at least 6%, by weight, of chromium and increasing its resistance to corrosion and abrasion which comprises subjecting said surface to an abrasive treatment t reduce the normal passivity thereof, forming in said surface a thin film of dark color comprising hydrated oxides of iron and chromium by treating the steel in a solution of sulphuric acid, water and an etching-inhibiting-oxidizing agent selected from the group of acids consisting of chromic, vanadic, metavanadic, manganic and permanganic, drying said film and thereafter dehydrating said oxides by heating the steel to a temperature of the order of 800 to 1700? F., and cooling, to harden said. surface and increase its resistance to abrasion.

4. The method of hardening the surface of a corrosion-resisting alloy steel containing at' least 6%, by weight, of chromium and increasing its resistance to corrosion and abrasion which comprises subjecting said surface to an abrasive treatment to reduce the normal passivity thereof, forming in said surface a thin film of dark color comprising hydrated oxides of iron and chroof a corrosion-resisting alloy steel containing at least 6%, by weight, of chromium, a dead, nonrefiecting, dark color, hardening said colored surface and rendering it highly resistant to corrosion and abrasion which comprises first reducing.

the normal passivity of said surface by blastin it with a mixture of abrasive and water, forming in said surface a film of dark color comprising hydrated oxides of iron and chromium by immersing the article in a solution of water, sulphuric acid and an etching-inhibiting-oxidixing agent of the group of acids consisting of chromic, vanadic, metavanadic, manganic and permanganic, drying said article, and thereafter hardening said colored surface and enhancing its resistance to abrasion by heating said article to a temperature of about 800 to 1200 F., to effect a dehydration of said oxides; the sulphuric acidwater solution being of such acid concentration,

in the absence of the inhibiting agent, as quickly to attack and etch the steel, and the quantity of inhibiting agent present in the coloring solution being at least sufficient to prevent the etching of the steel when treated therein.

6. The process set forth in claim 5 in which the sulphuric acid constitutes from about 25% to about 65%, by weight, of the sulphuric acid-water solution.

7. The process set forth in claim 5 in which the sulphuric acid constitutes from about 25% to about 65%, by weight, of the sulphuric acidwater solution, and the inhibiting agent constitutes from about 8% to about 15%, by weight, of the whole solution.

8. The method of hardening the surface of a corrosion-resisting alloy steel containing at least 6%,-by weight, of chromium and increasing its resistance to corrosion and abrasion which comprises firstforming in said surface a thin film of dark color comprising hydrated oxides of iron and chromium by treating said steel in a solution of sulphuric acid, water, and an etching-inhibiting-oxidizing agent selected from the group of acids consisting of chromic, vanadic, metavanadic, manganic and permanganic, drying said film, and thereafter dehydrating said oxides by heating said steel to a temperature of from about 800 F.v to about 1200 F. to harden said surface and increase its resistanceto abrasion; the sulphuric acid-water solution being of such acid concen tration, in the absence of the inhibiting agent, as

quickly to attack and etch the steel, and the quantity of the inhibiting agent present in the treating solution being at least suflicient to prevent the etching of the steel when treated therein. a

9. The method of hardening the surface of a corrosion-resisting alloy steel containing at least 6%, by weight, of chromium and increasing its resistance to corrosion and abrasion which comprises first forming in said surface a thin film of dark color comprising hydrated oxides of iron and chromium by treating said steel in a solution of sulphuric 'acid, water, and an etching-inhibiting-oxidizing agent selected from the group of acids consisting of chromic, vanadic, metavanadic, manganic and permanganic at a temperature between about F. and about 220 F., drying said film, and thereafter dehydrating said oxides by heating said steel to a temperature of from about 800 F. to about 1200 F. to harden said surface and increase it resistance to abrasion; the sulphuric acid-water solution being of such acid concentration, in the absence of the inhibiting agent, as quickly to attack and etch the steel, and the quantity of the inhibiting agent present in the treating solution being at least sufficient to prevent the etching of the steel when treated therein.

CLEMENTS BATCHELLER. 

